In combined gas and steam power plants, the gas turbine is firstly driven by the combustion of gas. Steam for a steam turbine is generated with the waste heat of the gas turbine. When the gas and steam power plant is started up, the gas turbine is therefore operated first. The steam turbine cannot be activated until sufficient steam is made available. In the case of single-shaft systems the gas turbine and generator are permanently connected to a shaft. The steam turbine is arranged on the same axis and can be connected via a clutch. It is therefore necessary to couple the steam turbine and gas turbine.
In practice, the coupling angle arises randomly here. EP 1 911 939 A1 discloses selecting the coupling angle in a targeted fashion. In this way it is possible to select a coupling angle at which the vibration loading is minimized. Roughly speaking, it is therefore possible to compensate unbalances of the two turbines to a certain extent. Particularly in comparison with a clutch in which both turbines are coupled in such a way that unbalances add together it is therefore possible to achieve a reduction in the vibration loading. Despite this advantage this method is not used.
A method for the targeted acceleration of the steam turbine with an acceleration value which is derived from the setpoint rotational speed difference and which is formed as a function of the detected differential angle, the acceleration and of the desired target coupling angle, is known from EP 2 813 675 A1, also WO 2014/198649 A1.
WO2015/124332 A1 discloses a method for coupling a steam turbine and a gas turbine having the following steps: 1) Acceleration of the steam turbine up to an output rotational speed which is below the rotational speed of the gas turbine; 2) Detecting a differential angle between the gas turbine and steam turbine; 3) Accelerating the steam turbine with an acceleration value which is derived from the setpoint rotational speed difference and which is formed as a function of the detected differential angle, of the acceleration and of a desired target coupling angle.
US 2015/059347 A1 discloses a method for coupling two shafts, in particular a gas turbine and a steam turbine. The rotational speed and rotational angle of the first shaft are detected, and the second shaft is controlled by adjusting a rotational speed of the second shaft relative to that of the first shaft. In this context, the acceleration of the second shaft is controlled in such a way that marks on the second shaft are at a predefined angle with respect to the marks on the first shaft as soon as the second shaft is adjusted to the predefined rotational speed. At a speed which is just below the synchronization speed, the method waits until the relative angle matches. The second shaft is then accelerated quickly and briefly and coupled to the first shaft.